Aggregation phenomena micelle formation

Aqueous solutions of ILs have some unique and important characteristics on the one hand, demixing leads to liquid-liquid equilibria instead of solid precipitation (like in the case of traditional inorganic salts) on the other hand, hydrophilic ILs that are totally miscible with water can form electrolyte solutions ranging in concentration from pure IL to pure water. In this context, we have performed a study on the determination of volumetric or volume-related (compressibility) data that will allow a more direct understanding of the relations between the self-aggregation phenomena (micelle formation in this case), the concomitant structural changes that occur in the aqueous solutions, and the corresponding consequences at a volumetric level. 

As indicated above, an important characteristic of a surfactant in solution is its solubility relative to the critical concentration at which thermodynamic considerations result in the onset of molecular aggregation or micelle formation. Since micelle formation is of critical importance to many surfactant applications, the understanding of the phenomenon relative to surfactant structures constitutes an important element in the overall understanding of surfactant structure-property relationships. 

A similar multiphase complication that should be kept in mind when discussing solutions at finite concentrations is possible micelle formation. It is well known that for many organic solutes in water, when the concentration exceeds a certain solute-dependent value, called the critical micelle concentration (cmc), the solute molecules are not distributed in a random uncorrelated way but rather aggregate into units (micelles) in which their distances of separation and orientations with respect to each other and to solvent molecules have strong correlations. Micelle formation, if it occurs, will clearly have a major effect on the apparent activity coefficient but the observation of the phenomenon requires more sophisticated analytical techniques than observation of, say, liquid-liquid phase separation. 

Formation and Structure of Middle Phase Microemulsion. The 1 - m - u transitions of the microemulsion phase as a function of various parameters are shown in Figure 4. Chan and Shah (31) compared the phenomenon of the formation of middle phase microemulsion with that of the coacervation of micelles from the aqueous phase. They concluded that the repulsive forces between the micelles decreases due to the neutralization of surface charge of micelles by counterions. The reduction in repulsive forces enhances the aggregation of micelles as the attractive forces between the micelles become predominant. This theory was verified by measuring the surface charge density of the equilibrated oil droplets in the middle phase (9). 

The quantity of insoluble substance which can be solubilised in micelles depends to a considerable extent on the chemical structure of the surfactant and is influenced by the presence of other components, which may influence either the micelle formation concentration (CMC) or the micelle geometry (aggregation number, shape). The transition from solubilisation to another important phenomenon, the formation of a micro-emulsions, is continuous. Microemulsions form spontaneously, whereas typical solubilisation systems attain their equilibrium state often only after extreme long periods of intensive mixing of both phases. 

When surfactants dissolve in water at low concentrations they exist as monomers (ionic surfactants are dissociated). As the concentration is increased aggregates termed micelles are formed. These appear at a well-defined concentration known as the critical micelle concentration (CMC). This is not a critical point in the sense of modern physics since micelle formation occurs over a very narrow range of concentrations. This range is so small that for almost all practical purposes it can be represented by a specific value, the CMC. For pure single surfactants below the CMC, all of the dissolved surfactant exists as monomers, while above the CMC all added surfactant forms micelles. With surfactant mixtures the phenomenon is more complex because the components usually have different individual CMCs, but the same considerations apply [2]. 

Sams et al. [61] proposed a two-state kinetic model which assumed a monomeric state and an associated state consisting of aggregates in various sizes larger than the monomer. The model describes only the fast process and assumes that the rate constants for association and dissociation are independent of the micelle size. A revised version of the two-state model [62,63] assumed micelle formation to be an adsorption phenomenon, with micelles at equilibrium with monomers adsorbing and escaping from the surface of micelles. 

From our experiments we cannot completely exclude the possibility of micelle formation of CHP in water. We found however little evidence for such micelles. The viscosity and light absorption of CHP solutions for example, are normal. We conclude therefore that no aggregates are formed in PMA-CHP solution that contain more than one PMA molecule. Our results can be quite well explained by the molecular model proposed by Lovrien [12]. With our results however a more refined mechanism can be formulated. Upon increasing the concentration of chrysophenine in an aqueous solution of PMA, the binding to the macromolecular surface increases. It reaches a critical level around a CHP concentration in solution of 0.001 M. A further increase of the CHP concentration leads to a level of bound dye that forces the polymer globule to open up to some extent. Thus new sites for dye binding become available, etc. Potentiometric titration is a more sensitive technique for recording the start of this process than is viscosity. However either technique demonstrates the cooperative nature of the phenomenon. 

True lipases show the interfacial activation phenomenon in their catalytic activity pattern. At low concentration of water-insoluble substrates, lipases are almost inactive, and the hydrolytic activity does not increase linearly. At a certain substrate concentration, however, the hydrolytic activity of lipases increases rapidly and the lipase kinetics resembles normal enzyme kinetics. This boost in activity is related to the formation of water-insoluble substrate aggregates such as micelles or another second phase. Only when this second phase is present, do lipases become fully active. This interfacial activation is caused by a large conformational change in the 3D structure of the lipases. In their water-soluble form, the active site is covered by a lid, which prevents the substrates from reaching it. At the lipidAvater interface, the lid is opened and the active site is accessible to the substrates. In addition, the now accessible area is mainly hydrophobic, which gives the open-form lipase the shape and behavior of conventional surfactant molecules with a hydrophilic and a hydrophobic moiety in one single molecule. 

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